The services between adjacent layers in the OSI architecture are expressed in terms of primitives and parameters. A primitive specifies the function to be performed, and the parameters are used to pass data and control information. The actual form of a primitive is implementation dependent. An example is a procedure call. The layout of Stallings DCC8e Figure 2.10a suggests the time ordering of these events. For example, consider the transfer of data from an (N) entity to a peer (N) entity in another system. The following steps occur:
The source (N) entity invokes its (N –1) entity with a request primitive including needed parameters,such as the data to be transmitted and the destination address.
The source (N –1) entity prepares an (N –1) PDU to be sent to its peer (N –1) entity.
The destination (N –1) entity delivers the data to the appropriate destination (N) entity via an indication primitive, which includes the data and source address as parameters.
If an ack needed, destination (N) entity issues a response primitive to its (N –1) entity.
The (N –1) entity conveys the acknowledgment in an (N –1) PDU.
The acknowledgment is delivered to the (N) entity as a confirm primitive.
This sequence of events is referred to as a confirmed service, as the initiator receives confirmation that the requested service has had the desired effect at the other end.
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